1. Field of the Disclosure
The disclosure relates generally to optical communications systems and parallel optical transmitters and, more particularly, to parallel optical transmitters having a power-monitoring module with a laser array, photodiode array and integrated lens assembly.
2. Brief Description of Related Technology
Communication systems continue to experience increased demand in the number of systems installed and in the amount of data such systems are called upon to transmit. Increased usage of the Internet and e-mail, as well as increased usage of mobile handsets and corporate intranets/extranets, have all resulted in dramatic increases in the amount of data throughput for communication systems. The bulk of this data traffic is routed through the optical networking infrastructure used by local and long-distance carriers, Internet service providers, and increasingly by companies building their own internal communications infrastructure.
Some communication systems transmit data through techniques that allow for simultaneous communication between users, typically using one of a variety of different types of optical transceivers. These optical transceivers commonly include an optical transmit portion that converts an electrical signal into a modulated light beam that is coupled to a first optical fiber, and a receive portion that receives an optical signal from a second optical fiber and converts it into an electrical signal. Other implementations employ one fiber for both optical signals, traveling in opposite directions.
Most recently, parallel optical transceivers have been developed where data is transmitted and received simultaneously over an array of optical fibers connected in parallel. These parallel optical transmitters offer more robust operation over other optical transceivers and may be used over longer distances, where there is a premium on avoiding crosstalk and other errors. A typical parallel optical transceiver consists of a vertical cavity surface emitter laser (VCSEL) array, and a PIN diode array. A parallel optical ribbon fiber is inserted into the optical transceiver, coupling to the VCSEL array or the PIN diode array.
For consistent operation, VCSEL lasers require constant or near constant monitoring of power levels, and in some cases in the monitoring of output channel wavelength. This is because VCSELs, as well as other laser sources, are susceptible to degradation of performance over a lifecycle (e.g., a reduction in optical power for a given drive current/voltage). VCSELs can also vary in performance, from device to device, although this is a lessened concern with VCSELs over other laser sources because VCSELs are generally considered easier to quality test post fabrication and less susceptible to design defects due to standard wafer, batch fabrication techniques used. In any event, these variables, and others, all contribute to the possibility that VCSEL operation will vary, either statically across an entire array as in the case of temperature dependence, or dynamically within an array as in the case of inter-channel optical power fluctuations.
To ensure acceptable VCSEL operation, power monitoring becomes integral to transmitter design of VCSELs. Typical power monitoring control is performed by measuring through the photodiodes the output of the VCSELs. While some laser sources can be designed to emit a monitoring beam separately from the main data beam, because VCSELs typically only emit light from one surface, any monitoring must be from the same output beam used for data communications.
Current attempts to monitor the power of VCSELS involve splitting off a portion of the modulated output light beam and measuring the portion through the photodiode array. While VCSELs performance can vary, so too can the performance of the photodiodes. Photodiodes are susceptible to varying operation under different environmental conditions, and they degrade with performance over time. This photodiode variability can be both linear and non-linear. In either case, the variability reduces the accuracy of the monitoring performed by these photodiodes.
There is a need for techniques that allow optical transceivers, optical transmitters, etc. to operate in a parallel optical module configuration while providing more accurate monitoring of laser conditions.